1. Field of the Invention
This invention relates to detection of failure conditions in high power electrical switching devices, particularly to the detection of high pressure conditions in high voltage vacuum interrupters, switches, and capacitors.
2. Description of the Related Art
The reliability of the North American power grid has come under critical scrutiny in the past few years, particularly as demand for electrical power by consumers and industry has increased. Failure of a single component in the grid can cause catastrophic power outages that cascade throughout the system. One of the essential components utilized in the power grid are the mechanical switches used to turn on and off the flow of high current, high voltage AC power. Although semiconductor devices are making some progress in this application, the combination of very high voltages and currents still make the mechanical switch the preferred device for this application.
There are basically three common configurations for these high power mechanical switches; oil filled, gas filled, and vacuum. These switches are also known as interrupters. The oil filled switch utilizes contacts immersed in a hydrocarbon based fluid having a high dielectric strength. This high dielectric strength is required to withstand the arcing potential at the switching contacts as they open to interrupt the circuit. Due to the high voltage service conditions, periodic replacement of the oil is required to avoid explosive gas formation that occurs during breakdown of the oil. The periodic service requires that the circuits be shut down, which can be inconvenient and expensive. The hydrocarbon oils can be toxic and can create serious environmental hazards if they are spilled into the environment. Gas filled versions utilize SF6 at pressures above 1 atmosphere absolute. Leaks of SF6 into the environment are not desirable, which makes use of the gas filled interrupters less attractive as well. If an SF6 filled interrupter fails due to leakage, the resulting arc can generate an over pressure condition, or explosive byproducts which can cause breach of containment and severe local contamination. Another configuration utilizes a vacuum environment around the switching contacts. Arcing and damage to the switching contacts can be avoided if the pressure surrounding the switching contacts is low enough. Loss of vacuum in this type of interrupter will create serious arcing between the contacts as they switch the load, destroying the switch. In some applications, the vacuum interrupters are stationed on standby for long periods of time. A loss of vacuum may not be detected until they are placed into service, which results in immediate failure of the switch at a time when its most needed. It therefore would be of interest to know in advance if the vacuum within the interrupter is degrading, before a switch failure due to contact arcing occurs. Currently, these devices are packaged in a manner that makes inspection difficult and expensive. Inspection may require that power be removed from the circuit connected to the device, which may not be possible. It would be desirable to remotely measure the status of the pressure within the switch, so that no direct inspection is required. It would also be desirable to periodically monitor the pressure within the switch while the switch is in service and at operating potential.
Perhaps at first blush it may appear that measurement of pressure within the vacuum envelope of these interrupter devices would be adequately covered by devices of the prior art, but the reality of the circumstances under which these devices operate has made a practical solution of this problem difficult to achieve prior to this invention. A main factor in this regard is that the device is used for controlling high AC voltages, with potentials between 7 and 100 kilovolts above ground, and extremely high currents. This makes application of prior art pressure measuring devices very difficult and expensive. Due to cost and safety constraints, complex high voltage isolation techniques of the prior art are not suitable. What is needed is a practical method and apparatus to safely and inexpensively measure a high pressure condition in a high voltage vacuum device, such as an interrupter, preferably remote from the device, and preferably while the device is at operating potential. It would be of further interest to be able to monitor the pressure status of these vacuum devices while they are powered down, on standby, or in storage prior to use.
FIG. 1 is a cross sectional view 100 of a first example of a vacuum interrupter of the prior art. This particular unit is manufactured by Jennings Technology of San Jose, Calif. Contacts 102 and 104 are responsible for the switching function. A vacuum, usually below 10−4 torr, is present near the contacts in region 114 and within the envelope enclosed by cap 108, cap 110, bellows 112, and insulator sleeve 106. Bellows 112 allows movement of contact 104 relative to stationary contact 102, to make or break the electrical connection.
FIG. 2 is a cross sectional view 200 of a second example of a vacuum interrupter of the prior art. This unit is also manufactured by Jennings Technology of San Jose, Calif. In this embodiment of the prior art, contacts 202 and 204 perform the switching function. A vacuum, usually below 10−4 torr, is present near the contacts in region 214 and within the envelope enclosed by cap 208, cap 210, bellows 212, and insulator sleeve 206. Bellows 112 allows movement of contact 202 relative to stationary contact 204, to make or break the electrical connection.